Natural gas that is produced from subterranean reservoirs usually contains mixtures of hydrocarbon gases (principally methane and ethane) and it may contain appreciable quantities of nonhydrocarbon gases (nitrogen, helium, water vapor, carbon dioxide and hydrogen sulfide). For efficient transportation and processing of the natural gas, it is frequently necessary to remove one or more of the nonhydrocarbon components.
Water vapor contained in natural gas is often removed from the gas in the vicinity of where the gas is produced so that the gas can be more economically and efficiently transported, stored and processed. If the water is not removed, if may accelerate pipeline and equipment corrosion and it may freeze or form hydrates and thereby plug pipelines, valves and orifices.
In a well known and commonly used process for dehydrating natural gas, the wet gas is contacted with a glycol such as triethylene glycol (TEG) in a countercurrent absorption column. Water vapor is absorbed by TEG as the gas flows up the absorption column countercurrent to the glycol flowing down the column. The TEG loaded with water is then heated to remove the water from it, and the lean TEG is recycled for mixing with wet gas in a continuous process. Although TEG has a low vapor pressure at temperatures typically used in this dehydration process, a small amount of TEG vaporizes or is entrained, thereby becoming a trace component of the natural gas mixture.
Acid gases (principally CO.sub.2 and H.sub.2 S) present in the natural gas are removed to produce a "sweet" gas which will not interfere with further processing of the gas and will meet customer specifications. One process to remove acid gases from natural gas involves contacting the natural gas with a liquid solvent containing dialkyl ethers of polyalkylene glycol. One such polyether is the dimethyl ether of triethylene glycol. Mixtures of such polyether solvents are sold by Norton Company under the trademark SELEXOL.RTM..
In a process for removing acid gases from natural gas, the natural gas is mixed with a polyether solvent in a conventional countercurrent absorption column under superatmospheric conditions. The solvent absorbs most of the H.sub.2 S and CO.sub.2, and it absorbs essentially all of the triethylene glycol and other contaminants previously introduced into the gas. The solvent containing dissolved H.sub.2 S and CO.sub.2 may then be regenerated by flashing in a series of flashing steps, followed by stripping to remove H.sub.2 S, and then recycled to the top of the absorption column for reuse. For economic reasons, it is desirable to recycle as much of the solvent as possible, thereby minimizing the need to add fresh solvent.
One problem with recycling the polyether solvent is that contaminants such as triethylene glycol (from upstream dehydration) and corrosion inhibitors (used in the transportation of the gas through pipelines) accumulate during extensive recycling of the solvent. The accumulation of these contaminants may significantly reduce the efficiency of the solvent to remove acid gases. In addition, some of the contaminants may precipitate out of the solvent and deposit on process equipment such as pumps, piping and heat exchangers. This loss of efficiency can increase the operating expense of acid gas removal and it may decrease the gas capacity of the gas processing facility.
Distillation removal of triethylene glycol from polyether solvents such as SELEXOL.RTM. is difficult because triethylene glycol has approximately the same vapor pressure as some of the components of SELEXOL.RTM..
There is a need for a simple process for removing contaminants from a polyether solvent to regenerate the solvent for longer usage. Removal of contaminants will reduce the need to periodically replace contaminated solvent with fresh solvent.